Brazilian small boat + 90hp merc. carb. - Help

[alexdante]

Staff would like to help one of you with my boat.
The boat is 15ft bassboat and supports engines up to 115hp.
Today I am using the prop 13x7/8x26p signature by RON HILL
The motor is 90hp mercury carb. 2011.
I jack plate 10" hydraulic bobs machine.
My best result was 55.1 miles at 5700 rpm.
This result was the 2800 ft to sea level.
http://i89.photobucket.com/albums/k220/alexprime/P8240469.jpg
http://i89.photobucket.com/albums/k220/alexprime/IMG_0759.jpg
http://i89.photobucket.com/albums/k220/alexprime/P8240463.jpg

I would like to raise the RPM to at least 6500rpm.
I bought a BIG FOOT Nose Cone Kit and want to install it but never installed as I'm looking for some photos over the Internet to have a better idea.

What you could direct me to improve my performance!

Best regards

[zul8tr]

I assume you are after more top speed and will sacrifice other performance for this gain?

Based on the data you provide and using a 2.33:1 gear ratio for the 90hp Merc the theoretical top speed for your rig would be 60.2 mph. That equates to about 8.5% slip. I would say that is pretty close to optimum for the rig but some improvement might be gained. Since you do not say where the prop shaft centerline is relative to the boat bottom and what engine angle you are running there could be improvement there by raising the engine to reduce drag and kick out to further lift the bow.

Also props are as different as _______ so try others. Perhaps less diameter for less hole shot but more top speed due to a bit more rpm. Even more pitch might work if the engine can go higher.

Higher elevation makes for less HP so reduced carb jetting is needed to compensate for less oxygen depending on the altitude above sea level. In your case at 2800 ft I think the standard jetting Merc used is probably OK for up to 3000 ft but you could do research on that with Mercury.

As usual be careful with these high speed runs :eek:. I assume you wear a good life jacket and helmut! ! :) :cool:

[alexdante]

I assume you are after more top speed and will sacrifice other performance for this gain?

Based on the data you provide and using a 2.33:1 gear ratio for the 90hp Merc the theoretical top speed for your rig would be 60.2 mph. That equates to about 8.5% slip. I would say that is pretty close to optimum for the rig but some improvement might be gained. Since you do not say where the prop shaft centerline is relative to the boat bottom and what engine angle you are running there could be improvement there by raising the engine to reduce drag and kick out to further lift the bow.

Also props are as different as _______ so try others. Perhaps less diameter for less hole shot but more top speed due to a bit more rpm. Even more pitch might work if the engine can go higher.

Higher elevation makes for less HP so reduced carb jetting is needed to compensate for less oxygen depending on the altitude above sea level. In your case at 2800 ft I think the standard jetting Merc used is probably OK for up to 3000 ft but you could do research on that with Mercury.

As usual be careful with these high speed runs :eek:. I assume you wear a good life jacket and helmut! ! :) :cool:

I measure the height from the pad.

Later put the measure.

best regards

[alexdante]

1" 1/2 below pad.
http://i89.photobucket.com/albums/k220/alexprime/11-2abaixopad.jpg

Position the TRIM
http://i89.photobucket.com/albums/k220/alexprime/PosiodoTRIM.jpg

best regards

[zul8tr]

The picture of the 1.5" depth you measured is difficult to determine how you are measuring it. I assume you used a straight edge laid on the botton of the pad and extended toward the rear of the motor and then measured vertically down to the center of the prop shaft at the end where the nut secures the prepellor so you grt the vertical distance from the pad bottom to the center of the end of the prop shaft.. Is that what you did?

Also the trim gage indicates that you are tucked under a bit.from neutral.You need to try trim toward the UP position.

[alexdante]

http://i89.photobucket.com/albums/k220/alexprime/11-2posiolateral-1.jpg

I put the platform of anti-cavity parallel to the pad.

then took the measure.

Hope that made ​​sure

"Also the trim gage Indicates that you are tucked under bit.from neutral.You need to try to trim Toward the UP position."

I do not understand what you mean

best regards

[zul8tr]

The 1.5" depth measurement clarification is what I thought you did. To reduce drag you can try raising the engine up say 3/4" with the same prop and note the rpms and speed and high speed cornering with more or less slip. In straight runs if rpms increase and speed remains essentially the same the prop is either in transisition zone and no benifit gained or you need more pitch to take advantage of the reduced drag on the lower unit and prop.

As for the trim gage I assume that it has been calibrated to read zero trim or prop shaft level with the bottom pad when the needle points to the center (third hash mark). If not calibrate the trim gage. In the picture the needle appears to be pointing off center toward the DN or down position which is tucked in or prop shaft lower at the end compared to the the front of the shaft.


When testing try different trim angles and note those as well as speed and rpm and other items of interest in your notebook.

If you need more pitch that can be done to your present prop within limits. But I suspect at least an inch can be added with proper bending . If more needed another prop may be needed.

Post a picture of the prop.

[A/B Speedliner]

Pete
I looks like the prop is lying on the floor behind the engine.
David

[zul8tr]

Opps missed that! :) But a closer pic from several angles would be helpful .

[alexdante]

It is the prop of the ground

Other photos:
http://i89.photobucket.com/albums/k220/alexprime/P8280449.jpg
http://i89.photobucket.com/albums/k220/alexprime/P8280447.jpg

it an EAGLE by RON HILL.

the photo of the clock shows the position of TRIM I reached the 55.1 mile
http://i89.photobucket.com/albums/k220/alexprime/PosiodoTRIM.jpg

Best regards

[OUTBOARDER]

How much does it weigh with you and bait wells full ?

[alexdante]

How much does it weigh with you and bait wells full ?

Approximately 1000lb full.

[zul8tr]

I see that the prop hub has a noticably smaller diameter than the gear case bullet diameter. That is not the most optimum condition as seperated flow will occur aft of the gear case bullet and spoil the water flow to the inner portion of the prop. This will cause a reduction in performance but as to how much is open to debate, I think small but noticable compared to a hub that matches the gear case bullet diameter.

The prop has rear rake that will help lift the bow and that is good in combination with the trim control to reduce hull drag which is a major force at high speed.

Do you have the original prop for this engine?
Was it an exhaust thru the hub design?
If you tried it how did it perform?

A 1000 lbs is alot of weight to deal with. At speed this has to be supported by aero lift on the hull including air compression on bottom ( aero is 2nd largest force) + hydrodynamic lift on the hull bottom (largest force from water) + bouyancy (smallest force). The hull is not the greatest design to produce aero lift so alot has to be created by hydrodynamic lift and that is a disadvantage since this lift has water drag associated with it which the engine has to fight aganst (including lower unit and prop drag and aero drag) as speed increases. Note water drag is about 780 times aero drag for an equal area, speed and conditions so reduced water drag is the most important at the speeds you are going.

As I have stated in previous posts raise the engine and play with the trim and record the results and report them here.

[sabine river killer]

a lower unit from a 115 will have a ratio of 2.07 if you switch to a 115 lower unit the tall gear will give you more speed that works better than going to a propeller with more pitch

[zul8tr]

As usual $$$$ = speed and that is probably a good change but changing it out could be a more expensive option than playing with setup and props.at first. Let's see what he thinks about that?

[alexdante]

Do you have the original prop for this engine?
Was it an exhaust thru the hub design?
If you tried it how did it perform?

Previous tests:

black max 12x1/2x23p 50.5 miles 5600rpm - 8" jack plate
Laser 21p 50 miles 6000rpm - 8" jack plate
Turbo 13.25x23p 50.3 miles - 5200rpm - 8" jack plate
Ballistic 13.5x24p 53.1 miles - 5600rpm (blueprint by mark croxton). 10" jack plate

best regards

[alexdante]

a lower unit from a 115 will have a ratio of 2.07 if you switch to a 115 lower unit the tall gear will give you more speed that works better than going to a propeller with more pitch


As usual $$$$ = speed and that is probably a good change but changing it out could be a more expensive option than playing with setup and props.at first. Let's see what he thinks about that?

I had thought of using the ratio 2.07, because I will install a kit big foot bobs nose cone machine.

That way I can lift more and the engine will have more consequentimente RPM.

But my only fear is the RPM falls too much and not getting the speed you have now.

There is a formula to know that I would only turn to change the ratio?

best regards

[alexdante]

post the new test only at the beginning of movembro.

I'll be traveling.

I thank all

best regards

[alexdante]

good day

performed the first test after a long downtime.

I made some changes to the boat:

I added a spacer 2 "up with 12" total spacing.

I installed a lower unit worked by BOB'S MACHINE.

Go up the engine 1 / 2 "and arrive at the ultimate resource for height adjustment.

Result: 5700RPM - 55.2 miles.

The test time was small but I think I can get the next 56 miles.

My prop is 13x7/8x36p EAGLE by Ron Hill.

Thank you all